In recent years, mobile devices show remarkable development, and one can easily enjoy image works anytime and anywhere with a small image reproducing device, a display attached to a mobile phone, and the like. Further, image data is being more often downloaded or transferred by a small memory, and the demand for the mobile devices is expected to increase.
In order to enjoy high-quality image works with a small display such as a display attached to a mobile phone, the display is required to have sufficiently high definition.
Meanwhile, a light-emitting element (also referred to as an electroluminescence element or an EL element) which includes a light-emitting layer containing an organic compound between a pair of electrodes is capable of high-speed response and DC drive at low voltage, and can be manufactured to be thin and lightweight. Therefore, the light-emitting element is putting into practical use as a flat panel display element and a mobile display element.
An EL element includes a pair of electrodes and an EL layer containing a light-emitting substance, which is provided between the electrodes, and emits light when the light-emitting substance contained in the EL layer is excited by current flowing through the EL layer. Therefore, in order to obtain high light emission intensity of such an EL element, a corresponding amount of current needs to flow through the light-emitting layer, and power consumption is increased accordingly. Further, as large current flows, degradation of an EL element is accelerated.
In view of the above, a light-emitting element which includes a stack of a plurality of EL layers and thereby capable of emitting light with high luminance as compared to a light-emitting element including only one EL layer, when current having the same current density flows through each light-emitting element, is proposed (e.g., see Patent Document 1). In the light-emitting element disclosed in Patent Document 1, a plurality of light-emitting units (EL layers) is separated from each other by a charge generation layer.